Rheology of Oligomeric Sulfonated Polystyrene Ionomers

Ionomers are flexible polymers containing small amount of covalently attached ionic groups. Those polar groups tend to form ionic aggregates in low dielectric polymer medium due to their strong polar/ionic interactions. The ionic aggregates, serving as physical crosslinks, endow the ionomers with excellent physical and mechanical properties. Owing to the absence of chain entanglement, oligomeric sulfonated polystyrene (SPS) ionomers were selected as a model system to understand the dynamics of ionomers.

Firstly, oligomeric SPS ionomers with different degrees of sulfonation (p) and metal cations were prepared from two oligomeric PS precursors. The gel point of SPS ionomers was determined as p = p_c, corresponding to one ionic group per chain on average. Below the gel point, p < p_c, the ionomer behaves like a sol with delayed Rouse relaxation. Close to the gel point, p ~ p_c, characteristic power law relaxations occur with G'(ω) ~ G"(ω) ~ ω¹ for mean field region, G'(ω) ~ G"(ω) ~ ω^2/3 for critical percolation region, and G'(ω) ~ω², G"(ω) ~ ω¹ for terminal region. Above the gel point, p > p_c, ionomers show a plateau in the G', and the plateau modulus increases with increasing p but remains constant for all different metal cations. Based on the mean field theory of Rubinstein and Semenov, a reversible gelation model was developed to analyze and predict the LVE behavior of SPS ionomers. This model, with only two parameters, the Rouse relaxation time of the Kuhn segment τ₀ and ionic dissociation time τ_s, predicts well the LVE behavior of SPS ionomers with all different p and alkali cations. The obtained segment relaxation time, τ₀, increases linearly with the increasing p but remains nearly unchanged for different metal cations. The ionic dissociation time, τ_s, however, highly depends on the type of metal cation, serving as an indicator for the strength of ionic interaction. For alkali metal cations, τ_sincreases with the decrease in cation radius due to the increased Coulomb energy. While for alkaline metal cations, where one divalent cation is connected by two acid groups,τ_s increases with increasing cation radius possibly due to the restricted mobility of the larger divalent cations. For divalent transition metal cations, the τ_s is strongly related to the electron configuration of the divalent metal cation, especially the configuration of d orbital, which may be related to the formation of covalent bonds within metal salts.

Secondly, the binary ionomer blends, prepared from two SPS ionomers with the same molecular weight but different sulfonation degrees and metal cations, show identical gel point and similar power law relaxation behavior with that of neat ionomers. However, even at the same ionic concentration, the blends exhibit a longer terminal relaxation time but a lower plateau modulus due to their broader relaxation time spectrum, which is probably originated from the broader distribution of ionic groups along the ionomer chains. In addition, the comparison of the experimental data and model predictions for the ionomer blends indicates that the mixing of different alkali cations follows a simple mixing rule: the average ionic dissociation frequency, i.e. the reciprocal of τ_s, of the blend is the number average of the ionic dissociation frequency of the two component cations.

Thirdly, in order to understand the nonlinear rheological behavior, the steady shear and startup shear behavior of SPS ionomers were investigated with p < p_c, p ~ p_c, and p > p_c. When p < p_c, the ionomer exhibits typical shear thinning behavior. When p > p_c, evident melt fracture occurs after the Newtonian region. Only when p ~ p_c, shear thickening emerges. The shear thickening behavior is accompanied by the absence of nonlinearity in the first normal stress difference coefficient, and the appearance of strain hardening in the growth viscosity during the startup shear. The magnitude of shear thickening increases with the decrease of temperature, the cation radius, and the molecular weight of the PS precursor, due to the increased contrast between the ionic dissociation time τ_s and the Rouse relaxation of the Kuhn segment, τ₀.

Fourthly, the LVE behavior of partially neutralized SPS ionomers with different sulfonation degrees (p) and neutralization degrees (x) was investigated and compared with their dielectric response. The x-independence of the plateau modulus and the similar LVE behavior between the partially neutralized SPS ionomers and the blends of fully neutralized SPS ionomer with unneutralized SPS ionomer containing the same amount of sodium indicate that both sodium sulfonates and sulfonic acid groups are associated in the same ionic aggregates and both contribute to the formation of the ionic network. In particular, the ionic dissociation time, τ_s, obtained from the reversible gelation model, is controlled by the neutralization degree x rather than the ionic concentration p. The increase of x enhances the of τ_s and zero shear viscosity η₀, especially near complete neutralization. Dielectric studies show that this rapid increase of τ_s and η₀ is related to the decrease of dielectric constant ε_s within the ionic aggregates due to the decreased amount of polar acid groups. The τ_s, predicted from the localized dielectric constant ε_s using Onsager equation, agrees well with those determined from LVE study. It indicates that the plasticization effect of the acid groups is to soften the ionic interaction between metal sulfonate groups through enhancing the localized dielectric constant.

Lastly, the effect of covalent crosslinking on the viscoelastic behavior of the zinc sulfonated EPDM/ zinc stearate (ZnSEPDM/ZnSt) shape memory compounds was examined under different temperatures and ZnSt loadings. The incorporation of ZnSt decreases the low-T creep compliance due to the irradiation induced degradation, but significantly increases the high-T creep compliance owing to the plasticization effect of ZnSt. The incorporation of covalent crosslinking significantly reduces the creep compliance of ZnSEPDM/ZnSt compounds, especially at high T and high loadings of ZnSt. After fitting with the Burger model, it was found that the covalent crosslinking could sufficiently reduce the plasticization effect of ZnSt through the suppression of the ion-hopping process. Therefore, the shape recovery performance of the ZnSEPDM/ZnSt can be greatly improved through reducing the irrecoverable creep compliance via covalent crosslinking.